In respiratory therapy where air is delivered to the mask under pressure, it is important to maintain a good seal between the mask and the patient's face. Leaks between the mask and the patient's face can reduce the desired air pressure in the mask and create increased noise. Both can reduce the effectiveness of, and compliance with, the therapy. In the first instance, the prescribed treatment parameters are not being maintained. In the latter, the increased noise can disrupt the sleep cycle of both the patient and the patient's bed partner.
Leaks are especially prone to occur as the patient moves during the night. Drag and movement of the air delivery tube or the mask system as the patient turns or moves can alter the positioning and alignment of the mask with respect to the patient's face, which movement can be translated or transferred to the cushion seal, creating leaks. Thus, while the mask may initially be leak free when attached to the patient, leaks are prone to develop later in the night as the patient moves in bed, awakening the patient. Hence, patients may tighten straps more than is necessary for pressure requirements in order to reduce or prevent leaks that result from movement.
Many different mask systems are known. One broad group of known mask systems include a rigid shell, a face-contacting cushion and headgear. The shell typically encompasses the nose or nose and mouth. Some known shells encompass the entire face. The cushion is typically constructed from a soft material such as silicone. A headgear provides a means to secure the mask in position. One known form of headgear consists of an arrangement of straps.
Certain mask designs have been developed to increase the flexibility of the mask cushion to enhance patient comfort while maintaining an effective seal between the mask and the patient. The Bubble Cushion (a registered trademark of ResMed, Ltd.) Mask, covered by U.S. Pat. No. 5,243,971, the subject matter of which is incorporated herein by reference, uses a flexible cushion membrane attached to a mask shell and the pressure inside the mask system to assist in the seal of the cushion membrane itself against the skin or face of the user.
The ResMed Mirage (a registered trademark of ResMed, Ltd.) Mask System, is covered by, inter alia, U.S. Pat. No. 6,112,746, the subject matter of which is incorporated herein by reference, has a contoured, three-dimensionally shaped cushion having an outer face-contacting membrane spaced apart from an inner frame rim to both assist in the seal and increase the comfort of the patient. Neither of these masks incorporates an expanded gusset section for mounting the cushion to the mask to assist in sealing the mask to the patient (user).
A known fitting procedure with a known mask has been to supply the maximum air pressure to the mask that will be supplied to the mask during the therapy and to adjust the strap tension to the necessary level to prevent leaks at that maximum air pressure. However, in many therapy regimens, this maximum air pressure is often encountered only during a portion of the duration of the therapy and the mask air pressure is lower at other times during the therapy. Such is the case, for example, when using auto-titrating or variable pressure systems or during ramp-up when using CPAP systems. Thus, the strap tension is higher than necessary during significant portions of the therapy duration. Further, since leaks are disruptive of both the sleeping cycle and the prescribed therapy regimen, patients will often tighten the straps even more than is necessary to prevent leaks at the maximum encountered air pressure. In known masks, this higher than necessary strap pressure directly results in a higher than necessary force of the mask cushion on the patient's face, particularly as the pressure goes below the maximum mask air pressure.
See FIG. 1, which shows a force diagram for a known mask 110 having a cushion 130 attached to a rigid shell 120. The cushion 130 includes a face-contacting portion 134 attached to a cushion sidewall 173. The cushion sidewall 173 can be relatively flexible, as in the ResMed Bubble Cushion® mask, or relatively rigid, as in the ResMed Mirage® mask. Although the mask 110 would be in contact with the face 42 of a patient 40 (shown in phantom) in use, for purposes of clarity in this diagram (as well as the diagram of FIG. 10), a flat foundation 43 is substituted for the patient's face 40. The total force of prior art masks on the user's face Fm has been found empirically to be given by the equation Fm=Fc+FAc, where Fc is the force of the cushion on the patient's face and FAc is the force on the patient's face of the mask air pressure P inside of the perimeter of Ac, the area of contact of the cushion with the patient's face. The force FAc is given by the equation FAc=PAc. Since the force Fc is distributed around Ac and is not merely located at two points on the cushion, as it might seem due to the limitations of the two-dimensional representation of FIG. 1, this force is shown in parentheses. Although Ac is shown inward of the sidewall 173 as would be the case if the mask 110 had just been brought into contact with the user's face with a minimal contacting force, in practice, the face-contacting portion 134 tends to roll under when sufficient force is applied to the mask 110 to seal the mask to the user's face such that Ac can expand outward, toward the sidewall 173.
The force (tension) in the headgear strap FS for the prior art mask has been found empirically to be given by the equation FS=(Fc+FAc)/(2 cos θ), where θ is the angle of the head strap with respect to the mask 110. Thus, the force of the cushion on the patient's face Fc is given by the equation Fc=2FS cos θ−FAc. The force of the mask cushion on the patient's face is difficult to distribute completely evenly around the cushion in known masks, especially at higher forces, and results in localized high pressure spots around the mask cushion. This higher force on the face, and especially the localized high pressure spots, are uncomfortable to the patient and can disrupt the sleep cycle. See, for instance, FIG. 2, which charts the force required to secure a mask on a face versus the air pressure in the mask (measured in centimeters H2O). As seen there, the force required to maintain a known mask sealed to the face throughout a mask air pressure range is most substantially affected by the maximum air pressure in the mask that will occur during therapy. That is, the force of the mask on the face remains at a fairly high level even when the pressure in the mask drops and this force of the mask on the face is directly related to the force necessary to seal the mask at the maximum mask air pressure. Misalignment of the mask will move the curve upward as higher forces are required to seal the mask to the face in light of the misalignment.
This force on the face increases as the head straps of a known mask are tightened to increase the sealing force of the mask, and thereby compressing the cushion and bringing the shell of the mask closer to the patient's face. When the straps are tightened, the shell of the mask moves a distance X between a position X0 when a seal is first obtained to a position Xp when it reaches the point where the cushion is being compressed beyond its normal range. The mask shell may be able to move beyond Xp but the cushion tends to become rigid or nearly so at about Xp, thereby limiting further travel. In a known mask, Fc generally increases at a first rate (i.e., the slope of the curve) as the mask moves toward the patient's face within a given range of X. This first rate occurs within the range of flexibility of the face-contacting portion 134. This first rate is also a function of the pressure in the mask acting on a back side of the face-contacting portion 134 of the mask. Thus, as the pressure in the mask increases, the first rate also increases within the given range of X.
However, the known cushion becomes less flexible as it is further compressed beyond such range of X. In a mask having a more flexible sidewall, as discussed above, Fc will then increase at a faster rate as the mask moves toward the patient's face due to a spring-force imparted by the sidewall until such point as the sidewall is nearly completely compressed and directly passing on the force from the rigid mask shell to the face. In a mask having a more rigid sidewall, as discussed above, Fc will then increase at an even faster rate for a short distance as the mask moves toward the patient's face but will quickly reach a point where the rigid sidewall is directly passing on the force from the rigid mask shell to the face.
See FIG. 3, which charts the force required to secure a mask on a face versus the movement of the mask frame (shell) from a relaxed positioned toward the face. The solid curve of FIG. 3 shows the force on the face for a known mask, such as the ResMed Mirage® mask, at a mask air pressure of 10 cms H2O. It can be seen from this curve that the force on the face increases in generally linear proportion to the movement of the mask towards the face within the range of flexibility the cushion 130. However, at such point where the cushion is nearly completely compressed (at approximately 5-7 mm in FIG. 3) so that the generally rigid sidewalls of the cushion 130 begin directly transferring the force from the rigid shell 120 to the face, the force on the face increases dramatically as the mask shell moves toward the face.
U.S. Pat. Nos. 5,492,116 and 5,655,527 to Scarberry disclose a full-face respiratory mask. The mask includes a flexible seal member 18 directly attached to a mask shell 12 and is attached to the user's head by head gear 24. The flexible seal member 18 itself contacts the user's face with a broad area of contact and maintains a seal with the user's face through pressure in the space 62 acting directly upon seal membrane inner surface 54.
Japanese Provisional unexamined patent application (Laid-open Kokai) published Jan. 6, 1999 entitled NASAL MASK FOR RESPIRATION, Provisional Publication No. 11-397 discloses a bellows-formed elastic body between a mask shell and cushion. As seen in the figures of that publication, the bellows portion of the mask projects an area over the patient's face that is substantially the same as the contact area defined by the line of contact of the mask cushion with the patient's face. This publication teaches nothing about the relationship between the area of the bellows and the force applied to the patient's face. Although the bellows provides limited mechanical flexibility, no significant pressure advantages or significant mechanical flexibility can be achieved. This mask does not overcome the sealing problems incurred by movement of the mask with respect to the patient's face without utilizing an increased head strap pressure across the mask air pressure operating range.